There is a class of instrument that measures the coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or may impinge on a retroreflector target that is in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Exemplary systems for determining coordinates of a point are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. A device that is closely related to the laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface. The laser tracker and laser scanner are both coordinate-measuring devices. Exemplary laser scanners are described in U.S. Pat. No. 7,430,068 to Becker et al., incorporated herein by reference, and U.S. Pat. No. 7,847,922 to Gittinger et al., incorporated herein by reference. It is common practice today to use the term laser tracker to also refer to laser scanner devices having distance- and angle-measuring capability. There is also a hybrid category of instruments known as total stations or tachymeters that may measure a retroreflector or a point of a diffusely scattering surface. An exemplary total station is described in U.S. Pat. No. 4,346,989 to Gort et al. Laser trackers, which typically have accuracies on the order of a thousand of an inch and as good as one or two micrometers under certain circumstances, are usually much more accurate than total stations or scanners. The broad definition of laser tracker, which includes laser scanners and total stations, is used throughout this application.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
Compensation parameters are numerical values that are stored in software or firmware accessible to the tracker. These numerical values are applied to raw tracker data to improve tracker accuracy. Initially, the manufacturer of the tracker finds the compensation parameters by performing measurements called compensation procedures. Later, the tracker will be used at the customer's site to make measurements. Periodically, the tracker will be checked for accuracy by performing interim tests. If the accuracy is substandard, the tracker operator will perform one or more compensation procedures on the factory floor. These can take from a few minutes to an hour or more, depending on the particular tracker and on the tests that are required. In most cases, the main cause of reduced tracker accuracy is thermal drift, although mechanical shock can also be important.
Compensation parameters generally relate to physical characteristics of the instrument. In examples given hereinbelow, some of these compensation parameters relate to (1) offset of a laser beam with respect to a mechanical point of rotation (gimbal point), (2) angle of a laser beam with respect to a line drawn perpendicular to two mechanical axes, and (3) non-squareness of two mechanical axes. Many other types of compensation parameters are used, but generally these compensation parameters (also called kinematic model parameters or simply parameters) relate to physical characteristics of the instrument. Hereinafter the terms compensation parameter and parameter are taken to be synonymous.
When a laser tracker is powered on after having been off for a significant time, it warms up as a result of the heat produced by the motors and the internal electronics. After a period of time, typically on the order of an hour or two, the tracker reaches a stable equilibrium temperature, if the ambient temperature is stable. After warm-up is complete, standard metrology practice calls for compensating the instrument, followed by an interim test procedure to verify that the compensation was successful. After compensation and the test procedure have been completed, the tracker is ready to take measurements. If the compensation procedure is performed on the laser tracker before the tracker is fully warmed up, the compensation parameters will continue to change as the tracker continues to warm up. As this happens, compensation parameters become less and less accurate and the measurement results degrade correspondingly.
From a practical point of view, the time taken for warm-up, compensation and testing is lost time, because the tracker is not available to take measurements. For this reason, it is standard metrology practice to keep the tracker powered on continuously whenever possible. This eliminates the warm-up period and assures that the tracker is ready to take measurements at any time.
In many real world situations, however, it is not possible to keep the instrument powered on continuously. For example, the instrument may need to be stored or transported to another job site, or the user may simply want to conserve energy. In such cases it is not possible to avoid warm-up. In these cases, the best that one can hope for is to minimize the amount of time lost, both for the instrument and for the user.
The warm-up scenario places the user in a difficult situation. On the one hand, there is a need to minimize the amount of time that is lost waiting for the instrument to warm-up. On the other hand, there is a need for accuracy in subsequent measurements. This tradeoff is faced by laser tracker users every time they power up their instruments.
The difficulty is exacerbated by the fact that every warm-up sequence is different. The detailed behavior depends on the initial temperature distribution within the tracker, the ambient conditions, and the idiosyncrasies of the individual instrument. The initial transient behavior and the final steady state behavior are obscured to some extent by random noise and long term drift. Also, while the long term behavior is roughly steady state, there is an element of subjectivity when a human operator decides whether the instrument is “close enough” to a steady-state value. In other words, the detailed behavior of a laser tracker during warm-up is complex, and determining when the tracker is warmed up is a non-trivial exercise.
Perhaps the greatest limitation of present methods is that there is no guarantee that the user possesses sufficient skill and knowledge to make the warm-up determination correctly, which can lead to many errors.
To some extent, the way the tracker is mounted may help to reduce the required warm-up time. An example of such a way of mounting a tracker is given in U.S. Published Application No. 2010/0195117 to Easley et al. However, this mounting method does not provide a method for determining how long to wait before the tracker is warmed up.
There is also a need to check the stability of an instrument such as a laser tracker even if the instrument is already warmed up. One reason for checking instrument stability is to ensure that performance is consistently being met. It is particularly convenient if such a method of checking the instrument is performed automatically. In addition, if the instrument does not have the expected absolute performance or stability, it is highly desirable to have a method to diagnose the cause of the decreased performance or stability. It may also be desirable to provide a “paper trail” to auditors demonstrating that the laser tracker was warmed up or stable when used.
What is needed is an accurate, objective, quantitative, and possibly automated method to determine when an instrument is warmed up or stable.